In response to the recent demand for fuel efficient tires, rubber compositions incorporating silica have been used in treads and various other tire components. Silica, on the surface of which there are hydrophilic silanol groups, shows lower affinity for the rubber components, particularly natural rubber, polybutadiene rubber, styrene-butadiene rubber, and other rubbers generally used in tires, than carbon black, and thus tends to provide inferior abrasion resistance and mechanical strength (e.g. tensile strength, elongation at break).
Many techniques for solving the above problem have been proposed, such as for example introducing a modifying group reactive with silica into a rubber component, or using a silane coupling agent to enhance the interaction between a rubber component and silica. However, common silane coupling agents have only a limited effect on dispersion of silica because, unfortunately, the functional groups in the molecules react with each other and aggregate before the reaction with silica. Patent Literature 1 discloses the use of a highly reactive mercapto silane coupling agent and the combined use of silane coupling agents. These methods still have room for improvement in terms of fuel economy, rubber strength, handling stability, wet performance, and other properties.
Moreover, not only fuel economy but also abrasion resistance are demanded these days from the standpoint of protection of resources. Thus, the use of fine particle silica having high reinforcing properties has been proposed. However, fine particle silica is usually very difficult to disperse in rubber compositions and cannot be dispersed well. As a result, unfortunately, aggregates are left, and thus abrasion resistance and mechanical strength cannot be greatly improved, or physical properties may even deteriorate in some cases. Such problems are more likely to occur especially with polymers having functional groups introduced therein because these polymers themselves also have poor processability. Therefore, there is a need for techniques that achieve a balanced improvement in properties including fuel economy, abrasion resistance, wet performance, and handling stability.